1. Field of the Invention
The present invention relates to the field of electrical testing and, more particularly, to devices for testing the response of geophones.
2. Description of the Art
Seismic mapping of subterranean geologic features involves the use of a seismic energy source and reception by seismic detectors. When used on land, the source of seismic energy can be an explosive charge which is detonated in a bore hole located at a selected point in a plot of land, or can be another energy source which delivers a series of impacts to the surface of the earth. For marine mapping, the energy source can be an explosive charge, an airgun, an imploder, or the like, which creates pressure waves. Acoustic waves from an energy source move through the earth and are transmitted back from pronounced strata boundaries; transmitted waves reach the surface after varying time intervals, depending upon the distance and nature of subsurface materials traversed.
The returning acoustic waves are detected by devices called geophones (on land) or hydrophones (for marine use), which convert the waves into electrical signals. In seismic mapping, it is common to use numerous detectors, electrically interconnected in series, parallel, or series-parallel arrangements called "strings." It is also common to use multiple strings, called an "array," to receive the acoustic waves.
Typical geophones used in seismic exploration consist of a coil of wire, suspended by springs in the field of permanent magnet. Seismic waves which impact the geophone cause a relative motion between the coil and magnet, thereby generating an electrical signal, the characteristics of which are related to the amplitude and frequency of the waves. The geophones are manufactured to close tolerances, to obtain a maximum degree of uniformity in their electrical signal outputs for a given mechanical input. However, signal characteristics tend to change in a random manner over time, as the geophones are handled and used repeatedly.
Due to these changes, methods have been devised for determining response characteristics of the geophones. One method involves mounting a geohpone to be tested on a shake table, adjacent a "standard" geophone. The two geophones are vibrated and their outputs are compared. Another test involves applying a steady-state oscillating signal to a geophone and measuring the signal generated by the geophone, which opposes the applied signal.
A more comprehensive test can be obtained by applying a constant DC current to the geophone, which creates a magnetic field in the coil and causes displacement of the coil away from its resting position within the permanent magnet. Removing the applied current causes the magnetic field in the coil to collapse, permitting the coil to return to its original resting position, which return will include certain oscillations about the resting position due to the influence of the suspending springs. Movement of the coil within the field induces an electrical signal in the coil; this signal can be compared to signals generated by other geophones which receive similar DC test voltages. This testing method yields what is generally denoted the "step function response" of a geophone.
However, when the magnetic field in the coil collapses, a rather strong voltage is rapidly formed in the coil, independently of the less rapidly generated test signal due to movement of the coil. This voltage interferes with recording and processing operations for the step function test signal.
U.S. Pat. No. 4,392,213 to Kung et al. shows the use of a delay period, after displacing the coil with an applied voltage, before the geophone-generated step function signal is recorded. This technique allows the strong interfering voltage to decay before the desired information is acquired, greatly simplifying test data acquisition and processing. The apparatus of Kung et al., however, is complicated, utilizing clock, timer, and relay circuitry to perform geophone step function testing.